The biological recognition technique is an emerging identity recognition technique. In order to realize stable recognition, most biological recognition techniques (including face recognition, iris recognition and 3D face recognition) use the near infrared imaging technique, namely, an infrared light source is used to actively illuminate biological features, and a camera technique is used to receive digitized near infrared images of the biological features obtained by reflection of the infrared light source so as to make recognition.
Iris recognition is an emerging biological recognition technique, which is applied more and more widely in the field of identity recognition. Safe and convenient identity recognition is a challenge in offering services for mobile terminal-based business. Currently, using mobile terminals as the means for identity confirmation mainly depends on passwords and cards, which has problems such as hard to memorize, vulnerable to being stolen and less secure. Among various identity recognition techniques, iris recognition has the highest security and precision, and it has many advantages such as unique individual, requiring no memorizing, theft-proof and high security level.
Under the present state of art, in order to add the function of iris recognition on a mobile terminal (such as a mobile phone), a near infrared camera module needs to be added on the front side of the mobile phone, which is independent from the front-facing visible light camera module for selfie. That is to say, two holes need to be provided on the front panel of the mobile phone, one is for selfie and the other is for iris imaging, so the industrial design becomes complex and the appearance is not good-looking.
In the prior art, for people of different races with eye color of yellow, brown and black, the iris imaging design realized by the near infrared camera module generally uses imaging of the infrared light with a frequency spectrum band of 760 nm-880 nm; an extra infrared light source (between 760 nm-880 nm) is needed for light-supplementing illumination, and the near infrared camera module requires energy that can receive said infrared band.
The iris recognition employed on the mobile phone is mainly used for identity recognition of the user himself, so it is usually used in a front-facing manner for user experience; but the existing front-facing selfie camera on the mobile phone cannot receive the light source of said infrared band or receives it with great attenuation due to the coating filtration for the camera per se. Therefore, the iris recognition in the prior art requires a separate near infrared camera for iris imaging, while it cannot multiplex with the existing camera (e.g. existing front-facing color camera on a smart mobile phone) for visible light imaging (with a spectral frequency of 380-760 nm). As a result, the size of the iris imaging system increases greatly, the cost increases, the design becomes complicated, the user experience is not good, and the iris imaging system cannot be miniaturized to be integrated into mobile terminals that are more widely needed.
Therefore, a technical bottleneck at present is how to use one camera for two functions, i.e. the normal function of visible light imaging for front-facing selfie and the function of infrared imaging for biological recognition.
Multiplexing a single camera on the mobile device can achieve both the normal function of visible light imaging for front-facing selfie and the function of infrared imaging for biological recognition, and methods for realizing the dual band imaging of near infrared and visible light mainly include: adding a mechanically switchable infrared light filter in the imaging system (see China patent CN201420067432.X), using an optical filter having infrared and visible light dual band transmission spectrum (see U.S. Pat. No. 8,408,821, China patent CN104394306A), and an image sensor with pixels that can detect both visible and infrared light (see U.S. Pat. No. 7,915,652, China patent CN104284179A).
The mechanically switchable infrared light filter (see China patent CN201420067432.X) has a relatively large size, so it cannot be widely used on mobile devices. The U.S. Pat. No. 8,408,821 disclosed using a dual-bandpass optical filter to allow passage of the visible light and the infrared light at the same time, but in this case, the visible light part will be interfered by the infrared light and the images will become reddish, meanwhile, the infrared light part will be influenced by the visible light waveband and the precision of biological recognition will be affected; besides, the patterned optical filter is not used. The optical filter in the China patent CN104394306A includes a first region and a second region, the first region is a dual band pass coating which can allow passage of visible light and infrared at the same time, and the second region is an IR band pass coating, which can only allow passage of infrared light with a specific wavelength. Engineering implementation of such a method on a smart phone is problematic, because such a design will cause infrared light to enter into the first region to make the selfie reddish, moreover, the infrared light signal passing through the second region cannot completely correct the reddish effect, because the imaging areas of these two regions are inconsistent in the same frame, and due to the different photosensitive contents, factors for perfect correction cannot be determined stably to correct the problem of the reddish selfie, especially when the background color is green, thus the selfie effect becomes poor, which is unacceptable by users with demand of high quality front-facing selfie function of mobile phones; meanwhile, when the first region is used for infrared biological recognition imaging, since the visible light can also pass through it, the iris features, for example, will be affected by light spots, such as bright spots reflected by spectacles, lamplight and windows, reflected by complex visible light from the external environment. In the design of the multi-region double-spectrum imaging device, either the quality of the selfie or the performance of infrared biological recognition will be affected, and it is impossible to have it both ways. Especially, the biological feature imaging will be influenced by visible light from the complex external environment, which results in deterioration of the quality of the collected biological feature images, and the recognition precision of the back end algorithm and the user experience will be severely disrupted. The problem of axial chromatic aberration of visible light and infrared imaging is not considered, so the image quality of selfie will be affected, besides, it only mentions to use the voice coil motor to adjust the focal point, without providing the method of realizing auto zoom for double-spectrum imaging. The U.S. Pat. No. 7,915,652 relating to infrared light only relates to the design of an image sensor for double-spectrum imaging, but it does not provide the design for the entire imaging system. The multi-spectrum optical filters for different regions as used in the China patent CN104284179A) are a color filter array attached to the surface of the image sensor, rather than independent optical filters. In addition, this design does not give consideration to the problem of axial chromatic aberration of visible light and infrared imaging, so it is difficult to collect near infrared iris images that meet the requirements for recognition at the normal distances of use for mobile devices